737s, these are harder. Older 737-200s have different engines, they are smaller and more attached to the wing. on 200s the cockpit windows are a bit different to. The 300/400/500 are the trickey ones. Dont let the higher the number, the bigger confuse you. The 300 is the longest, the 500 is a bit shorter, and the 400 is the shortest. they dont have wingtips, like the NG. the 6/7/8/9 73s can be difficult to. the 6/7s early modles dont have wingtips, but the latter 7s do. the 8s and 9s both have wingtips and the 9 is longer than the 8.
As for MD, DC, and Airbus, I don't know to much about, but I can tell the difference between a DC and MD from the wingspan. Airbus are just ugly and have stange winglets that go up and down. Airbuses under bellys are different to.
I think you will find some 737-800's don't have winglets. Indeed I know this as fact, as I have flown on one.
I thought the -400 was longer than the -500? and roughly the size of a -700, though with a different nose and cockpit windows and tail.
One I used to have difficulty with was 737-800 and 767-200. untill I got the idea of the tail and nose
then the difficulty was 737-700 and -800. So I bought two models at the same scale of a -700 and an -800 and stuck them side by side in my bedroom and looked at them every day. It kinda sunk in after a while and now I can see the proportions quite easily. (did you know the wings of a 737-800 and -700 are the same size? the only difference is fuselage length really!)
I live in Australia and we only really have 3 (or 6 depending on how you count them) airlines.
Virgin Blue use only 737-700 and 737-800
Jetstar use 717-200 and A320
Qantas use 737-300, 737-400, 737-800, 767-300, 747-300, 747-400, A330-200, A330-300, with their Qantaslink subsiduary using 717-200, Dash8 Q300 and Q400, and in the west, I believe some BAe146's
Independant Regional Express Airlines REX (used to be part of the Ansett group, and now I think with an agreement with Virgin Blue) use Saab S340b and Merin Metro's, and possibly Twin Otters DHC6 too.
Since I'v gotten quite used to seeing these aircraft, and the King Air 300, around my international airport of choice, I can tell them apart quite readily. MD-11 too, although it's hard to mistake those, maybe except for against a DC-10 (but the 'tenner' doesn't have winglets)
the Airbusses are quite distinctive to me, the winglets of the A320, and the way it sits high on it's landing gear compared to a 737, and has a wierd nose compared to a 767.
A330 has an interesting wing-fuselage joining point. and the -300 is quite a bit longer than the -200. The nose and cockpit windows are also very unlike the boeings. seem alot more 'blended' and 'rounded'.
The landing gear of the 767 vs A330 is also quite interesting to see, particularly at landing or immediatley after takeoff.
the A340, I can't imagine confusing it with anything else. It's far larger than the 707 and DC9, and is quieter, and doesn't belch smoke
and if you confuse the A340 with a 747, well your just not trying hard enough
I would suggestwww.airliners.net
and if you get stuck on one pairing (for instance the 737 varients) well maybe buying a model or two might not go astray.